1. Field of the Invention
The present invention relates to photometric analysis of chemical processes and products.
More particularly, the present invention relates to photometric techniques for continuously monitoring the process efficiency and product strength of a chlorine dioxide generation plant.
2. Description of the Prior Art
Dilute solutions of chlorine dioxide are used by the pulp and paper industry as an extremely effective bleaching compound due to reactive perference thereof for lignin in lieu of cellulose. However, due to the explosive nature of the gas, it must be generated at the point of use and stored and used in dilute aqueous solutions.
Continuous generation of chlorine dioxide by means of the Mathieson Process, for example, follows a complex reaction series occurring from contacting a gaseous mixture of air and sulfur dioxide with a liquid solution of sodium chlorate and sulfuric acid in a sparging vessel. The process is highly exothermic and therefore the generator must be externally cooled to maintain a safe gaseous chlorine dioxide product temperature below 57.degree. C. (135.degree. F.).
The warm gas is drafted at 2 to 6 inches of water vacuum from a volumetric void where it is collected at the top of the gas/liquid contact sparging vessel into the bottom of a counterflow absorber vessel. Cascading chilled water, approximately 4.degree. C. (40.degree. F.), absorbs the upflowing gas into a dilute aqueous solution of approximately 8 to 9 grams per liter of water.
Although the material flow balance is adjusted to yield an end product of the desired solution strength, numerous events may upset the balance. For example, an increase in the absorber supply water temperature will reduce the absorption rate thereby permitting an undissolved portion of the generated gas to be exhausted to the atmosphere through the draft fan at the top of the absorber vessel. Although the end product solution strength is batch analyzed chemically at relatively frequent intervals, hourly for example, upsets of the nature described may develop in a matter of minutes. Consequently, it is highly desirable to maintain continuous surveillance over the process in both the gaseous and liquid stages.
In the past, no surveillance was exercised over the gaseous stage of chlorine dioxide generation. Since an aqueous solution of the compound will vary in color from pale yellow to yellow-brown in generally direct proportion to increased concentration, liquid concentration has been monitored by means of a colorimeter. However, large changes in concentration are required to produce a change in color hue of such magnitude as to be discernible by the colorimeter technique. Consequently, the technique has proven unsatisfactory due to insensitivity.
It is well known that chlorine dioxide is highly absorptive of infra-red spectrum light of greater than 7000 Angstroms wavelength. Such properties are exhaustively reported by authors A. H. Nielsen and P. J. H. Woltz in The Journal Of Chemical Physics, Volume 20, Number 12, December, 1952. Although laboratory apparatus has been devised to measure vaporous chlorine dioxide, such apparatus cannot be used to measure concentration of the compound in aqueous solutions since the water phase of the solution absorbs a significantly greater portion of the infrared spectrum than chlorine dioxide. Moreover, such vapor measuring apparatus is extremely environmental sensitive and consequently unsuitable for harsh industrial conditions.
Chlorine dioxide is also known to have an ultraviolet or blue spectral absorption band centered at approximately 3600 Angstroms. However, for reasons to be subsequently explained, this ultraviolet absorptive property of chlorine dioxide may not be readily exploited in an industrial environment for the present objective of concentration surveillance.
It is, therefore, an objective of the present invention to teach a method and apparatus for detecting small changes in the concentration of chlorine dioxide in dilute solution with water.
Another object of the present invention is to provide a method and apparatus for the surveillance of gaseous chlorine dioxide.
These and other objects of the invention will become apparent from the following description.